There are many instances in which unwanted mechanical vibrations propagate through an extended structure or assembly of structures or bodies in mechanical communication. One example is the transmission of vibrations from a spacecraft or satellite to antennae or other sensitive equipment or apparatus mounted from it at the end of a thin boom. Typically the boom will not be mechanically damped and the vibrations will be communicated along it to the antenna equipment or apparatus and so interfere with its normal functions.
Another example is the transmission of unwanted vibration from heavy machinery such as ship engines via pipes, mounts or other members connecting it to the supporting structure.
In U.S. Pat. No. 4436188 to Jones there is disclosed a controlled motion apparatus including three actuators disposed respectively along three orthogonal axes. However, in contrast to the present invention, the Jones apparatus is intended either to induce vibration simulation for the testing of equipment, or alternatively to produce the required movements for a machine tool such as a profile cutter.
Various publications deal with the cancellation of specific modes of vibrations by point-acting actuators. Examples are the article by Schaechter et al in the Journal "Guidance" I(5), 1984, at pages 527 to 534 and the article by Mace in the Journal "Sound and Vibration" 114(2), 1987 at pages 253 to 270. In copending U.S. patent application No. 07/492817, filed Mar. 13, 1990, now abandoned and replaced by co-pending U.S. patent application Ser. No. 07/809,704, filed Dec. 17, 1991 (corresponding to British published patent specification No. 2229789) a method is disclosed whereby mechanical vibrations of any type may be prevented from propagating along a beam, pipe or other extended part of a mechanical structure.
It is an object of this invention to provide a method of and a system for active vibration control whereby vibrational displacements at a chosen point or region of an extended mechanical structure may be at least reduced, as desired, or even caused to disappear altogether.